Advancing Ghouls
by SilversunnyD
Summary: In a world of Titans and Ghouls, one is bound to be overshadowed and forgotten. So while mankind has been fighting titans, the ghouls have been simply watching from the sidelines at what might be the extinction of the human race. However, their time of sidelines may be coming to a close.
**This is a collaboration with a friend. We traded messages back and forth for days over many things for this particular story. We both enjoy Attack on Titan and Tokyo Ghoul. While I know more about Tokyo Ghoul, she knows more about Attack on Titan. So please give credit to my friend Useful. I probably wouldn't have posted this without her telling me to. X)**

 **Disclaimer: I clearly do not own Attack on Titan or Tokyo Ghoul. Because I would find a way to make a crossover like this an actual thing if I did.**

* * *

Underneath the walled civilization was another civilization. The Underground where pretty much anything goes. Murder, thievery, slave trade and much, much worse. It was a depressing place that most residents would likely never leave, dead or alive. No, they were the undesirables not allowed above.

A few cases had occurred in the past where military from above would come down for whatever reason. Although those cases were few and far, this was an exception. For today-assuming it was daytime-two military personnel walked the dark and dreary streets with many sets of wary eyes on them. Their particular military branch wasn't the sort to venture underground often.

The pair wore dark green cloaks with the emblem of their branch on the back. A pair of wings, blue and white. It was a symbol some had come to call the Wings of Freedom. That meant the pair were from the Survey Corps, the military branch that ventured outside the protective walls in attempts to reclaim the lost territory of humanity.

Now, what would they be doing in the underground? Well, they were looking for someone. Someone that would likely be leaving with them, if all went accordingly.

"Nagachika, are you sure about this?" The taller of the two cloak wearing Survey Corps members questioned.

"I'm sure he can help commander. We haven't talked much since Shiganshina fell, but I'm almost positive he'll help." Said the smaller of the pair reassuringly. "Ah, that's the coffee shop he told me about!" Suddenly he sounded a lot more excited as he pointed to a sign hanging over a coffee shop. "An-tei-ku. I believe that's how it's pronounced."

"And you're sure he'll be there?" Questioned the man called Commander.

Nagachika as he had been called, chuckled a little and shook his head. "He said if he's not there then to ask the manager."

They fell into silence as they approached the coffee shop, opening the door to be greeted by the light tinkle of a bell. For an underground shop, it was actually quite nice. A real wonder as to how anything in the underground could be so nicely furbished without even a scuff on the wooden floors. Perhaps a greater wonder was how anyone got enough money to open such a place.

There were tables along the walls, smartly placed away from the front window and there actually were several customers calmly sipping their coffee, some even eating actual food. It was a wonder how anyone could afford such in the underground, unless the food and drink were really cheap.

Nagachika pulled his hood down, revealing a head of dirty blonde hair and a pair of brown eyes. He had a grin on his face as he took in the sight of the coffee shop. His full name was Hideyoshi Nagachika, most called him Hide for short. "Whoah! This is a really nice shop! I bet even Levi would smile at the sight of this place!"

"Why thank you young man." Up until that statement, the elderly man behind the counter had gone unnoticed. He was smiling kindly as he washed a fragile looking tea cup.

"It is a nice shop." The Commander conceded as he too pulled down his hood, revealing a head of neat blonde hair. He had some rather distinctive eyebrows and nicely defined facial features. His name was Erwin Smith. "It must be quite costly." There was no missing the suspicious tone.

To the elderly man's credit, his smile never so much as twitched. "I can't take all the credit, my staff does most of the work. Now, how can I help you two? I get the sense you aren't here for coffee."

Before Erwin could speak, Hide quickly stepped up. "You must be the manager, I'm looking for my friend, Kan-err... Sasaki." He smiled nervously and scratched the back of his head. "Do you by chance know where I might be able to find him? It's really important."

"Oh?" The manager sounded a bit amused at the younger man's slip-up. "He's in back right now, checking up on our stock." He gestured to the door on his side of the counter. "First door on the right."

"Thank you sir!" Hide called back excitedly, having hopped over the counter and already gone through the door.

An exasperated sigh escaped the older member of the Survey Corps before he followed.

Behind the door was a nicely furbished hallway with numerous doors, further showing that the coffee shop apparently had very good business. Had he had time, the commander probably would've investigated all the rooms. As it was however, he was already running late for a meeting. All this business underground was taking a lot longer than he had initially expected.

Upon rejoining Hide, Erwin found him talking the ear off another young man who had black hair and wore an eye patch. Based off the description the ever hyper Hide had given him, this was the person they were looking for.

"-and that's why we're here!" Hide finished, although he'd been talking so much and so fast it would be a surprise if the new face had caught any of it.

Indeed, the black haired man had a frown on his face that said he had no idea what was going on. His single gray eye shifted to Erwin when the man entered the room.

"Ken Kaneki, or should I address you as Haise Sasaki?" Erwin questioned, not waiting for Hide to introduce them.

The man with an eye patch frowned, shifting his gaze over to Hide for a moment before looking at Erwin once more. "I'm Sasaki down here."

Erwin nodded, looking the black haired man over with a critical eye. "I'm commander Erwin Smith of the Survey Corps. Nagachika tells me you may be able to help us with a problem we've been having."

"A problem that I can help with? I serve coffee for a living, I don't know how I could be of help to the military." Sasaki, as he currently wished to be called, rubbed his chin before letting out a sigh. "Please, take a seat." He gestured to one of two couches in the apparent stock room.

The room, like the rest of the establishment, was nicely furnished. Smooth wooden floors, a collection of cabinets along one wall. Two bags of coffee beans sat innocently on a counter next to a coffee making set. In the middle of the room was two matching green couches with a coffee table situated between them.

"You might serve coffee now, but you were in the Training Corps before Shiganshina fell, correct? I found your file." Stated the Survey Corps Commander as he took a seat on one of the couches. "Although you were in an accident shortly before we lost that wall, correct? It's part of the reason you were counted among the dead, sounds like you were pretty gravely injured."

* * *

 _Two Years Previous_

 _It was a night of celebration for them. They had officially completed their time in the Training Corps and soon enough would be deciding on which military branch they would be joining. So, to celebrate they'd taken to the streets of Shiganshina and found a bar. By now most of them were rosy cheeked and in some cases delirious from mass alcohol consumption._

 _One of the more responsible cadets had taken to reading a book and forgone drinking for the most part. He had black hair and gray eyes and was casually dressed in a white shirt and dark slacks. Like most of the others he had forgone his uniform for the night so as not to upset the locals... well, not for being in the military anyways. His name was Ken Kaneki._

 _Looking up from his book to check on his rowdy comrades, he smiled slightly and released a light sigh._

 _"Such a loud bunch."_

 _Kaneki jumped, losing his grip on the book he had in hand allowing it to clatter to the floor. Not even thinking he quickly reached down to pick up his book, only for his hand to meet another's. Looking up, he found himself looking at the most beautiful woman he had ever seen._

 _She had unusual lavender hair and pretty purple eyes framed with expensive looking red wire glasses. Her dress was purple for the top with white for the skirt and she wore a simple pair of black shoes. Overall, she looked like the type one might have a chance of meeting in the innermost wall._

 _"Sorry, I didn't mean to startle you." She smiled sweetly before lifting the book and returning it to his hand._

 _"A-ah..." He was at a loss for words for a moment, sitting up straight and visibly blushing. "N-no... it's uh... alright?"_

 _The woman covered her mouth and giggled lightly._

 _"Yo! Kaneki! I thought you weren't drinking tonight!" An arm was slung over Kaneki's shoulder with such force that it almost knocked Kaneki out of his seat. "So why are your cheeks rosy?" There was Hide, with some of the reddest cheeks in the bar and a teasing grin on his face._

 _"H-Hide!" The rather unfortunate Kaneki was quick to shove his best friend off, his cheeks growing even redder._

 _"Oh?" Rather than taking offense at Kaneki's actions, Hide took notice of the woman. A grin spread across his face. "Ooooh, I see." Hide gave Kaneki a light elbow to the side before running off to rejoin the festivities._

 _By that point, Kaneki just wanted to bury his face into the book and never see the light of day again. Instead he settled for an exasperated sigh and lowered his head to hide his ever reddening cheeks._

 _There was another giggle from the woman. "I see, you're here with them."_

 _"Unfortunately." Came Kaneki's murmured response as he lifted his head._

 _She was still smiling, in his eyes, she might as well have been an angel with that smile. "I'm Rize by the the way."_

 _"Ken... but everyone calls me by my last name, Kaneki." He offered a weak and nervous smile._

 _"I noticed you were reading one of Sen's works. The Black Goat's Egg, I believe?" Her purple eyes rested on his book for a moment._

 _Immediately his smile grew to a real and ecstatic full blown smile. "Yes! I love all of Sen's works, especially this one!" He even held up his book for emphasis. Get Ken Kaneki talking about books, and you had a whole new Kaneki._

 _"Sen is an excellent writer." Rize admitted, laughing a little at Kaneki's sudden enthusiasm. "I love the unusual usage of words, especially in Monochrome Rainbow."_

 _"Monochrome Rainbow? That's Sen's most recent work, right?"_

 _She nodded almost eagerly, her smile growing a bit. "It came out about a month ago."_

 _At this revelation Kaneki suddenly looked depressed. "I suppose I've been too busy to keep up."_

 _"So you don't have a copy yet?" Rize questioned, tilting her head slightly. Her answer was a sad sigh and a negative shake of the head from Kaneki. "Well... if you want I know where you could get a copy."_

 _Kaneki's head shot up immediately. "Really?!"_

 _Rize giggled and nodded her head. "We can go right now even."  
_

 _At this, Kaneki paused at the suggestion. He glanced back towards where the rest of his unit were still celebrating... only to find most of them making kissy faces in his direction, causing his eye to twitch slightly. "I'm not sure I should be leaving these drunks alone..."_

 _"Hmm... It doesn't look like they'll be leaving anytime soon though." Urged Rize, gently placing her hand over his. "Besides, it's not that far from here. Just down the street."_

 _"Well..." He glanced back only to see further teasing from his comrades and sighed. "I suppose it couldn't hurt. Especially if it isn't that far anyways."_

 _Her smile got bigger and she grabbed his hand. "Great! Let's go!" She didn't even wait for him to say anything, they were out of the bar before he even had the chance._

 _The streets of Shiganshina were dark and the only source of light were lamps at the doors of various shops and some faint light given off by the moon. Before entering the bar the sun hadn't even so much as begun to set. It made Kaneki wonder just how long they'd been at the bar now and he figured he'd have to ask everyone be cut off when he got back._

 _After getting the book from a shop that was surprisingly still open at the time, the pair had taken to walking, chatting excitedly about the books written by their favorite author, Sen. In fact, Kaneki had been talking so excitedly, he hadn't noticed they had taken a turn that did_ not _take them back to the bar. In fact, the street they were on now appeared to be getting rebuilt from the ground up. Not surprising given how old some buildings were. However, when Kaneki noticed it was a surprise._

 _Looking around and taking note of his odd surroundings, he turned his gaze to Rize. "Hey... where are we going...?"_

 _"I'm sorry." Rize wrung her hands nervously, looking down. "I was kind of hoping you'd walk me home... it's just, there's been a few murders in the area recently and I..."_

 _Kaneki stepped up so he was facing her directly. "It's alright." He reached towards her hands hesitantly. "I don't mind."_

 _"I'm so relieved." Rize leaned in close so that her lips were right next to his ear. He could literally feel her breath. "After all, now that we're this close..."_

 _His breath hitched, not knowing what she would say next. He was seeing red now. Was he about to faint from how close they were right then?_

 _"I won't have to worry about you making too much noise."_

 _There was a splash as blood spattered onto the ground. It took a long moment for Kaneki to register what had just happened. Time seemed to stop as his gaze slowly lowered to his stomach, where something glowing and bright red had pierced him. The book slipped from his hand, going completely unnoticed._

 _"Hmm..." A hand traced the length of his right arm in an almost thoughtful manner. "You're more muscular than I expected. Book worms are usually softer."_

 _That was Rize's voice, but when he looked up he saw something he'd never so much as heard of before. The sclera of her eyes had turned black, the pupils a bright red and there were slightly pulsing red veins around her eyes. A small splatter of blood on her left cheek. His blood._

 _He could only stare at her uncomprehendingly. What was he looking at? What was he seeing? What the hell was happening?_

 _"Oh well. I can still scramble your insides." She wiped at the blood on her cheek and tasted it. "Mmm... you do taste good anyways."_

 _The glowing red things retreated from his stomach, revealing to him that they were tentacles that seemed to be coming from her back._

 _Kaneki found that he couldn't stand on his own now without the killer support that had kept him standing until now. He started to stumble back, but he didn't even get the chance to fall before Rize plunged her hand into his already gaping wound. He couldn't even speak, all he could do was cough up blood._

 _"All you delicious humans hiding inside these big walls... you live in fear of the titans and don't even know there's more than them. You have no idea we ghouls even exist. It's so pitiful." She withdrew her hand and laughed. "But you sure are fun to play with."_

 _Suddenly, her tentacles slammed into his side with such force, sending him flying into a nearby scaffolding. Many building materials crashing to the ground. Nothing massive that could just end his torment however._

 _Rize approached slowly, her shoes clicking across the ground until she stood before him. A big grin on her blood covered face. It looking like it would all end here._

 _Then there was the creak._

 _She would look up only to see a pile of wooden beams falling off the top of the scaffold, coming down straight towards her._

 _There was a massive crash that someone was sure to have heard. However, Kaneki was already drifting off, falling unconscious._

 _"What's going on?!"_

 _"Where did all this blood come from?!"_

 _"Look! Over there!"_

 _"This doesn't look good! Someone, get Dr. Jeager!"_

* * *

Sasaki was quiet for a long moment before taking a seat on the opposite couch. "Yes, I was in the Training Corps. I joined with Hide. However I still don't see how I could be of any help." He rubbed his chin again before noticing Hide hadn't sat down yet. Instead Hide had moved to stand behind Erwin.

"I'll assume you know a bit about what the Survey Corps do and what goes on outside the walls. So I'll jump to the main reason we're here." Leaning forward, Erwin folded his hands together and looked Sasaki directly in the eye. "Some of our people have been going missing. Which is not entirely unheard of since we deal with titans more than anyone else. However the rate of those disappearing has been increasing. For some reason, Nagachika thinks you might know something."

* * *

 _Earlier That Week_

 _The results of their most recent expedition outside the walls... was depressing._

 _All the bodies they'd managed to recover were laid out for their families to collect. Unfortunately for Hideyoshi Nagachika, he was in charge of making sure the bodies went with the right families this time. Even the normally smiling Hide, found the sight saddening._

 _Boots against wood was what alerted him to another living presence._

 _Quickly standing up straight, Hide placed his right fist over his heart, left arm going behind his back. Seeing who had come, he eased out of the stance._ _"Commander Erwin, sir."_

 _"I see you make a habit of forgoing formalities, Nagachika." Stated the commander, not even sparing Hide another glance. His gaze was focused on their fallen comrades._

 _"Our losses only seem to be growing lately." Erwin murmured to himself, but Hide still heard it. Without another word the commander left._

 _Hide just frowned at the commander's back before turning his gaze to the fallen. They were all covered with sheets so that when the families came they wouldn't have to see all the gore if they didn't want to._

 _Normally, he would try to avoid looking at the dead. But what Erwin said about increasing losses only served to awaken Hide's investigative curiosity. So before the families could start arriving he decided to take a look at their wounds._

 _What he found was mostly partial bodies, the missing parts having been bit off. Most by titans, which wasn't too surprising. But what was surprising was when he took notice of the bites that weren't that of the titans. The telling factor was the bite marks, the outline the teeth had left behind._

 _A titan's teeth were large, easily able to bite a human in half. So a titan's bite was more often a straight line on a limb but more broken if it was say, a bite in the torso because more then one tooth would be needed._

 _The other bites were taken in small but in multiples to compensate, like when a human bit into a piece of bread. Small, human-like bites were on some of the dead. However, most humans had trouble breaking skin with their teeth, let alone drawing blood, not even thinking of taking a solid bite out of the flesh._

 _Something else came to mind however. Ghouls. In terms of size, they matched a human easily. In ability however, they surpassed humans easily. He knew they survived off human flesh as well._

 _Most people didn't know that ghouls even existed. He only knew because of his friend Kaneki, who until a year and a half ago he'd thought to be dead after the fall of Shiganshina. Turns out Kaneki had somehow ended up with ghoul organs that turned him into a half ghoul. It took until a year ago for Kaneki to actually admit to the existence of ghouls and what had happened to him._

 _This presented Hide with troubling thoughts. Did this mean there were ghouls outside the walls? Or were there ghouls among the Survey Corps?_

 _Well, if anyone knew it would be Kaneki. Technically Kaneki and other ghouls, but Kaneki would probably be the one more open with information._

 _If the Survey Corps was getting targeted by ghouls, and he had a gut feeling they were, then maybe Kaneki knew how to stop them. Or... better yet, maybe Kaneki himself could stop them somehow._

 _It wasn't like Kaneki could just leave the walls though. Maybe if he got Erwin involved somehow though, the commander could do something._

 _Hide would just have to be careful though, he couldn't just let slip that ghouls existed._

 _Play the cards right though and he might have Erwin's support and be able to bring Kaneki out of hiding._

 _... Although Ken Kaneki was currently operating under the name Haise Sasaki..._

 _He'd have a lot of a explaining to do._

* * *

There was a furrowing of brows and Sasaki turned his gaze to Hide, who was currently making motions behind Erwin. Imitating sneaking up behind the commander and taking a bite out of the man. Realization lit up Sasaki's eyes and he suddenly seemed to understand. When Erwin noticed the change in Sasaki, he looked behind him towards Nagachika, however Hide had already returned to a straight-backed standing position.

"... I see." Sasaki murmured thoughtfully, tilting his head from side to side in deep thought. "I do have an idea of what's happening... but there's not much that can be done."

"Well, what _is_ happening?" Erwin's eyes narrowed, the lack of information clearly getting on the man's nerves.

Sasaki averted his gaze from the commander and shook his head. "I'm afraid I can't say."

There was a tense but short silence broken by the Survey Corps commander. "My men are dying and disappearing on every expedition. If something is happening I want to know why."

"Look, I can't say I understand what you go through on those expeditions, but I can't tell you. I am sorry." Without looking at Erwin, Sasaki stood up and looked about to leave the room, but the commander's voice stopped him.

"Wait. Could you do something about the disappearances, since you have an idea what's going on?"

There was a pause and Sasaki returned his gaze to to the commander at last. "I suppose. But it's not like I can just up and rejoin the military."

"I wouldn't be so sure." Now Erwin was smiling, which was not a good sign. "Ken Kaneki was classified as dead after the fall of Shiganshina. If he were found alive, even if underground with an alias, he could be charged for desertion. His associates charged as well, for aiding and abiding a deserter."

Sasaki froze, a look crossing his face that was somewhere between panic and murderess. His mouth opened and closed but no words came out.

"Of course, I could alter the records, have them say your injury required a long period of rehabilitation. You would no doubt be able to readjust to the 3DMG pretty fast, you were among the top ten in your unit after all." Now there was a smirk on Erwin's face. "I'd hate to see potential go to waste."

* * *

 **If you have any thoughts or pointers, please let me know. Due to the fact I have a lot of other stories long overdue for updates, it may be a while until the next chapter I'm afraid.**


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